When a person (hereafter called “subject”) wishes to acquire spectacles, the first step is, naturally, to determine the characteristics of the lenses required for correcting their sight. This step is performed by an ophthalmologist. The next step is choosing a frame that matches the subject's wishes.
Lastly, the optician in charge of this person must take a series of measurements to adapt the spectacles (frame and lenses) to the subject's exact morphology, for actually manufacturing the spectacles fitted with the corrective lenses.
Among these measurements, it is clear that the interpupillary distance (called “PD” in the rest of the description for “Pupil Distance Measurement” or “PD Measurement”) is a key measurement, which is defined as the distance separating the centers of the pupils of both eyes (see FIGS. 1 and 3b) when the subject is perfectly head on, with the head straight and looking to infinity. It is also called “PD far” to characterize the fact that the subject is looking to infinity. In addition, optical practitioners also use a close-look measurement (“PD near”) and an intermediate measurement.
This interpupillary distance measurement PD effectively determines the relative distance between the optical axes of the lenses mounted in the spectacles. It is noted that each of these lenses is, in simplified fashion, geometrically conformed as the spaces comprised between two spherical caps having different centers.
Correct measurement of the interpupillary distance PD is the key to perfectly corrected vision. In contrast, incorrect measurement causes at least one of the lenses to be misaligned in relation to the optical axis of the eye located opposite the lens, resulting in imperfect vision and tired eyes.
The average distance between the centers of the pupils is 64 millimeters in adults, with 80% of the values falling between 62 and 68 mm. Accuracy of the order of 0.5 mm is required for correct vision, particularly for progressive lenses, which must be placed as accurately as possible in relation to the subject's eyes. It should be noted that with the gradual aging of the population, the proportion of progressive lenses in the total number of corrective lenses prescribed is increasing; it is currently over 20% of the total.
In general, the interpupillary distance measurement PD realized is the “PD far” performed with a pupilometer or by some other method. Thus, the supposed measurement is the distance between the lines going through the optical centers and the centers of the pupils and assumed to be parallel. This is based on four hypotheses: that the subject is truly looking to infinity; that the measurement instrument takes the orthographic projection into account; that it is sufficiently accurate; and that there is no vergency problem with the subject.
In practice, the expression of the PD distance takes the vergency of the subjects into account: for example, looking to infinity generally requires having the lines previously defined as divergent (non-parallel) because of the physical systems of the eye (cornea) and of perception (interior of the eye, brain).
In correlative fashion, two additional measurements are also desirable to determine the way the subject wears the spectacles, and the morphology of their face.
The first measurement is known as “mono-PD” and represents the distance between the center of the pupil and the center of the frame (which materializes the resting point of the spectacles on the nose) when the subject is looking to infinity.
The second measurement is called as “Fitting Cross Heights”, which is used in the context of progressive lenses (“varifocals”). This measurement is the height of the line linking the two centers of the pupils in relation to a reference level of the spectacles (e.g. the bottom of the spectacles). As is also understood, accurate measurement of this height is preferable to ensure the spectacles are suitable for correct vision to infinity.
There are several conventional methods for performing the interpupillary distance PD measurement. The interpupillary distance measurement can be realized either between the two eyes (binocular measurement) or by calculating the distance between the center of one pupil and the center of the spectacles (monocular measurement).
One of the methods of the previous state of the art is simply to measure the space between the edges and irises of both eyes with a ruler while looking in a mirror. This approximative measurement yields an accuracy of the order of 3 millimeters, which is adequate for conventional corrective lenses, but not suitable for progressive lenses.
Another solution, generally used by opticians, consists of placing the subject's face on a fixed mechanical reference frame, which comprises, schematically, eyepieces and a thumbwheel, and performing an optical measurement. This method, which is more accurate than the previous one, nevertheless has inaccuracies: a single subject can be found to have interpupillary distances PD that differ by a few millimeters, depending on which optician performed the measurement.
Yet another method consists of taking at least one picture of the subject and of a calibration object and deducing the interpupillary distance PD from the value measured on the picture and from corrective coefficients determined according to the image of the calibration object. This method also has the drawback of requiring a predefined position of the subject, which is difficult to obtain in practice.
Opticians realize the “Fitting Cross Height” measurement more-or-less empirically by observing the subject wearing spectacles.
In addition, these measurements assume that the subject is effectively looking to infinity, which is rarely the case and which can also disrupt the measurement by one to two millimeters.
Lastly, it is known that the human eye is not a sphere and that its movement is not modeled by simple rotation, but by a combination of rotation and translation; this means that the measurement must take the orientation of the gaze into account at the time of said measurement, without which it cannot be extrapolated to the normal position of looking to infinity.
It is understood therefore that measurement is made difficult not only by existing equipment and methods, but by the very nature of the human body and by the unavoidable movements of or errors in positioning the subject.
In current optical systems, a simple carrying over of the distances is used. In the same way, when using a photographic optical system: a measurable offset can be seen, due to the perspective of the camera rather than to morphology. For a better measurement, the tridimensional position and morphology of the subject's visual system must be known. For measurement purposes, experts agree that the eye's center of rotation is the focal point where images are formed.
There are three-dimensional measurement systems, but these are very invasive and need to be implemented in stores with sophisticated and expensive equipment.
Objectives of the invention The objective of this invention is to propose a method for measuring the interpupillary distance that remedies the problems of inaccuracy and measurement difficulties mentioned above.
According to a second objective of the invention, the method can be implemented at lower cost, which means wider utilization can be envisaged.
A third objective of the invention is to provide an interpupillary distance measurement in a very short period of time.
Another objective of the invention is also to be implemented without requiring calibration objects.
The measurement method can be implemented with equipment that is generally available with consumers and uses a protocol that can be understood by anyone: this avoids every user needing to go to the store.